Field of the Invention
The present invention relates to a dampening element for a bearing arthro-prosthesis.
It is known in the field that during the simplest dynamic phase like a slow pace, the stress that a bearing prosthesis has to sustain, especially the hip, reaches and goes beyond a load of at least five times the patient's weight. Under such conditions, at least two major problems arise with the implants of hip prostheses, that is, first, the steadiness of the implant at the prosthesis-bone interface and, secondly, the load dampening effect in the articulation of the limb. Given the state of the art, the implant steadiness may now be considered as positively achieved, both in the direct implant prostheses and in those having cement-mediate implant. However, the dampening of the load in the articulation has made so little progress that the stresses delivered from the head to the cotyle of the prostheses result of increasing and almost peak-growing intensity during one foot bearing, and so with hardly or no deformation of the joint.
The early attempts made to solve this problem consist of the interposition of a pad of biocompatible plastic material between the femoral head and the cotyle of a hip prosthesis, fully enveloping the prosthesis head thus making both the concave and convex surfaces of the pad smooth and of a constant thickness at any point of the pad. The results that have been achieved to date are completely negative owing to the very high and constant rigidity of the pad even under low loads due, in turn, to the non-compressibility of the material used. More recently, sheaths made of the same plastic material have been adopted partially enclosing the prosthesis head and providing the convex surface with through holes or, channels distributed according to the sheath meridians and parallels, communicating to each other with the intent being to decrease their capacity upon the crushing of the sheath under load. Also these known types of sheaths exhibit a very limited deformation in the load direction such that they do not perform the function of dampening the shocks to which the joint is subjected during the deambulation and more so, especially during low loads. The simulation tests that have been carried out have highlighted this circumstances which is confirmed by the fact that, after some time, the sheath exhibits a concavity due to wear in the zone of maximum stress.